Hot carbonates deep within the Chicxulub impact structure

Abstract Constraining the thermodynamic conditions within an impact structure during and after hypervelocity impacts is extremely challenging due to the transient thermal regimes. This work uses carbonate clumped-isotope thermometry to reconstruct absolute temperatures of impact lithologies within and close to the ∼66 Myr old Chicxulub crater (Yucatán, México). We present stable oxygen (δ18O), carbon (δ13C), and clumped-isotope (Δ47) data for carbonate-bearing impact breccias, impact melt rock, and target lithologies from four drill cores on a transect through the Chicxulub structure from the northern peak ring to the southern proximal ejecta blanket. Clumped isotope-derived temperatures (T(Δ47)) are consistently higher than maximum Late Cretaceous sea surface temperatures (35.5°C), except in the case of Paleogene limestones and melt-poor impact breccias outside of the crater, confirming the influence of burial diagenesis and a widespread and long-lived hydrothermal system. The melt-poor breccia unit outside the crater is overlain by melt-rich impact breccia yielding a much higher T(Δ47) of 111 ± 10°C (1 standard error [SE]), which likely traces the thermal processing of carbonate material during ejection. Finally, T(Δ47) up to 327 ± 33°C (1 SE) is determined for the lower suevite and impact melt rock intervals within the crater. The highest temperatures are related to distinct petrological features associated with decarbonation and rapid back-reaction, in which highly reactive CaO recombines with impact-released CO2 to form secondary CaCO3 phases. These observations have important climatic implications for the Cretaceous–Paleogene mass extinction event, as current numerical models likely overestimate the release of CO2 from the Chicxulub impact event.


Fig. S2 .
Fig. S2.Lithological and petrographical characteristics of representative carbonate phases within the upper part of the IODP-ICDP Expedition 364 impactite sequence.(A) SEM backscatter image of rounded foraminiferal calcite (Cal) grains, quartz (Qz) and phyllosilicates (Clay) in the upper part of the green marlstone (616.56 mbsf; modified from: (30)).(B) SEM backscatter image of the

Fig. S4 .
Fig. S4.Petrography of representative carbonate phases within impact melt rock interval 721.45 mbsf of the IODP-ICDP Expedition 364 drill core.(A) Cross polarized light (XPL) microphotograph image of a brecciated impact melt rock with a large microcrystalline melt clast (MMC) floating in a calcite matrix (721.45 mbsf).(B-C) SEM-EDS overview of the same brecciated impact melt rock sample displaying clear equigranular calcite, this sample is associated with the highest T(Δ47) value (327°C ± 33°C (1 SE)).

Fig. S5 .
Fig. S5.Petrography of representative carbonate phases within impact melt rock interval 726.21 mbsf of the IODP-ICDP Expedition 364 drill core.(A) Cross polarized light (XPL) microphotograph image of the contact between a green schlieren zone and microcrystalline impact melt rock (MM) (726.21mbsf).(B-C) SEM-EDS overview of the same sample showing that the cracks within the green schlieren zone are filled with calcite, grossular-rimmed andradite-garnet (Adr) and smectite-group clays (most likely dominated by saponite).

Fig. S11 .
Fig. S11.Isotopic cross relationships of Chicxulub core and impact ejecta material.(A) Relationship between δ 18 O data and average T(Δ47) data with the symbols referring to the lithological units from the four different drill cores (in grey), including the dataset of Burtt et al. (14) (with white symbols and highlighted with colored fields).Dashed lines are constant δ 18 Owater (VSMOW) values of possible diagenetic fluids based on the carbonate-water equilibrium relationships of (41).Note that Burtt et al. (14) used a different standardization and the T(Δ47) comparison here should be seen as relative, as it is not possible to recalculate the absolute temperatures of Burtt et al. (14) using the universally accepted I-CDES reference frame (62).(B) Isotopic cross plot showing δ 13 C (VPDB) versus δ 18 O (VSMOW) data with colors indicating the T(Δ47) data.Four Rayleigh trends are highlighted in this plot that represent fractionation during impact decarbonation as expressed by varying mole fractions of oxygen (Fo) (following(14, 42).The extent of the reaction (Fc) is plotted along one Rayleigh curve to show the extent of decarbonation.In addition, the K-Pg ejecta dataset from (14) is shown, in which the accretionary lapilli data largely plots within fields of Fo of ~0.1-0.2.This indicates that the formation of the lapilli was steered by decarbonation, but that the decarbonation process was not fully complete (14).The data from the current study plots largely above the Rayleigh curves, suggesting that decarbonation following atmospheric processes is not a major factor in explaining the isotopic trends and therefore,

Fig. S12 .
Fig. S12.Relationship between δ 13 C and CaCO3 content.Colors indicate the T(Δ47) data.The dashed line is representing the average δ 13 C signal for marine carbonate (~1 ‰ VPDB; (44)).Most of the high T(Δ47) samples plot above this value, suggesting that thermal impact processes are responsible for the more positive δ 13 C compositions in these crater lithologies.

Table S1 .
Overview of the analyzed Chicxulub samples and their stable oxygen, carbon, and clumped isotope data, together with their calcium carbonate content.